Configuration file format -- Variable Expansion

Attribute values can contain
references to other attributes. This is done similarly to how
Perl, Bourne shell, and Makefiles use variables. The variables
that are referenced will be expanded when the variable is used,
not when it is defined. This means that attributes don't have
to be defined in the order that they are used.

Example attribute reference in an attribute value:

database_base: ${database_dir}/htdig

or

database_base: $database_dir/htdig

Many default values use the attribute expansion mechanism to
make configuration easier and more generic.

In addition to variable expansion, it is also possible to
include the contents of a file as the value of an attribute.
This is done by putting the filename in backquotes (`).
Within the backquotes, variable expansion will still work so
that files can be specified relative to some path. The
backquotes and the filename will be replaced by the contents
of the file if it exists. All whitespace (spaces, tabs,
newlines, etc) in the file are condensed to one space before
the substitution takes place. The following are some examples
of the use of the backquoting scheme: